You have to use
:let @+ .= your_expression
see stackoverflow. The problem, as you have understood, is that you cannot capitalise the
For example, to add a line in your buffer to the clipboard, place the cursor on the line and yank it with
yy. Then type
:let @+ .= @0 to run (
:) the command
let to append (
.=) register (
@) 0, which always holds the last yank, to register
+ which is a representation of the clipboard.
For example, to add the string "abc" to the clipboard, type
:let @+ .= 'abc'.
(To change the X11 selection instead of the clipboard register
+ use register
vim has a huge set of commands. If you plan to adopt it as your favourite editor it is worth putting some effort into knowing, at least superficially, some of them. Most vim users probably only use about a dozen commands and are not interested it its true potential. It's a pity, as such tools could significantly reduce their workload.
You might like Learn Vimscript the Hard Way.
The builtin vim help is also viewable on the web at sourceforge , with this page on The Vim script language.